1. Field of the Invention
The desirability of forming composite materials into an integral unit has been a long-standing problem in the art.
It is of course desirable to form composite products for many reasons, i.e., to enjoy the benefits that each material contributes to the composite, to surround or imbed a relatively expensive material in a holder or carrier of less expensive material, to provide decorative or functional features in a composite material which are delineated by different colors, hardnesses, or other physical or chemical characteristics and the like.
In order to satisfy the need for composite products the prior art has resorted to various methods of manufacturing such composites. These attempts include the separate forming of the individual elements and assembly into a composite by means of an adhesive or mechanical fastener. This technique is relatively labor intensive requiring the skill of an operator to align the preformed elements in the position desired in the final composite.
With the advent of moldable materials it became possible to co-mold, either sequentially or simultaneously, various layers or parts of a composite product. It also became possible to place preformed molded parts or elements in a mold cavity and cast or injection mold flowable material about the elements so as to form a composite product. Although such techniques are generally known in the molding art, there are considerable disadvantages to forming composite products by the known molding techniques.
Placement and retention of the preform elements in a precise location in the mold cavity requires the use of projecting pins or depressions to retain the preformed elements in their desired position during the molding process. Such positioning means increases the cost of mold fabrication. Additionally the known positioning means are not sufficient to prevent "flash" or a "skin" of flowable material from penetrating between the preform and mold surface resulting in a defective composite product.
In the case of forming elastomers, such as silicone rubbers into a composite product, it would have been expected that the placement of a preformed insert, which is only partially cured into a mold and flowing a moldable material thereagainst would improve the bond strength between the preformed elastomer and the flowable material. This expectation is based in part on the theory that the partially cured elastomer would still contain sites which could cross-link with the flowable elastomer. However, in practice the use of partially cured elastomers as the preform does not result in a satisfactory bond with the flowable elastomer even though the composite material may may be fully post-cured out of the mold.
Furthermore, incompatibility of the various elements of the composite product may make adherence of the elements at their interface difficult or impossible to achieve even in the mold. Vapors entrapped in the preform or generated during contact of the molten material with the preform can deteriorate the strength of the "weld" or "bond" at the interface of the materials. Some incompatible materials can be improved by the use of an adhesive or primer on the preform to increase the adhesion of the elements. In order to assure adherence of preforms to the molded material, the prior art has also resorted to the use of mechanical means, such as projections or undercut depressions so as to provide a "mechanical interlock" between the preform and molded material. Although improving the strength between the individual elements in the composite, the provision of mechanical interlocks, primer and/or adhesive applications on the preformed elements increases the cost of production of the composite. Thus these prior art attempts to make composite products have not proved to be entirely satisfactory.